This invention relates to communication systems, and more particularly to communication systems subject to noise and interference.
In a spacecraft-based cellular communication system, such as those described generally in U.S. Pat. No. 5,717,686, issued Feb. 10, 1998 in the name of Schiavoni; U.S. Pat. No. 5,875,182, issued Feb. 23, 1999 in the name of Hatzipapafotiou; U.S. Pat. No. 5,907,541, issued May 25, 1999 in the name of Fairholm et al.; and allowed patent application Ser. No. 08/961,938, filed Oct. 31, 1997 in the name of Hudson, the spacecraft produces multiple partially-overlapping antenna xe2x80x9cspotxe2x80x9d beams, within which the various user terminals are located. Many of the user terminals are mobile, and from time to time may be found variously near the center of a spot beam in a location where the signal strength is great, and near the overlap region between beams, where the signal strength is weaker. Since the available spectrum is limited, the maximum possible service is provided by frequency re-use among the various spatially diverse spot beams. That is to say, that ideally the operating frequency band of a given spot beam is different from the operating frequency band of adjacent spot beams, but may be the same as the operating frequency band as semi-adjacent spot beams. The angular separation of antenna spot beams is selected to provide an estimated acceptable level of interference attributable to coupling of same-frequency-band signals between antenna beams. Nevertheless, these estimates are based on statistical measures, which may not be applicable to particular channels at particular times or channel loadings, with the result that some channels may experience interference in excess of the desired or acceptable amount.
A method for maintaining the quality of a signal transmission over a signal path, according to an aspect of the invention, includes the step of sensing the strength of at least a portion of the received signal resulting from traversal of the path in a first direction. The signal strength or level is compared with a predetermined threshold and the signal is deemed not to have faded if the signal level remains above the threshold, and to have faded if the signal level drops below the predetermined threshold. The signal quality or bit error rate of the received signal is sensed or determined, and the signal is deemed to be of poor quality if the quality drops below a predetermined threshold. If the signal level is indicative of a fade, and the signal quality is poor, the poor signal quality is deemed to be due to the fade, and the effects of the low signal level are mitigated by maintaining the same information rate while increasing the coding gain. For example, doubling of the bearer rate while maintaining the information rate allows one to use more coding bits, which results in an increase of coding gain. When the faded signal is found in one link direction, such as at the user terminal or at the gateway, the coding gain need be increased only in the faded direction; it often occurs, however, that the fade occurs in both directions, and it may be desirable to increase the coding gain in both directions of propagation. In a preferred embodiment of the invention, identification of fading in either direction results in an increase in coding gain in both directions of propagation. If the signal level is above the threshold and thus not indicative of a fade, but the signal quality is poor, the poor signal quality is deemed to be attributable to interference.
When interference is detected in this manner, at least one of the slot and the frequency of the channel are changed, while maintaining the same bearer rate and coding, with the expectation that changing of the channel slot or frequency, or both, as may be the case, will reduce the interference toward the designed values.
When an interference condition is found in one link direction, the slot andor the frequency need only be changed in that particular link direction. However, as in the case of fading, interference in one direction of propagation is often accompanied by interference in the other direction of propagation, so a preferred embodiment of the invention contemplates changing slot andor frequency for both directions of propagation.
In one particular mode of the inventive method, the step of sensing the signal quality includes the step of determining the signal bit error rate. In a particular embodiment, the bit error rate is determined by the technique of re-encode-and-compare.
In one context of the use of the invention, the pair of stations includes a gateway and a user terminal, and the signal path includes a spacecraft. In this context, the step of determining the strength and determining the signal quality are measured, for the forward link direction, by the user terminal, and index signals representing a band of the error rate and of the signal level are reported to the gateway. Likewise, the step of determining the strength and the signal quality are measured, for the reverse link direction, by the gateway. The processing for deeming the presence or absence of fade and of interference may be performed at either end of the path, but is preferably performed at the gateway, for both the forward and return links, using the index signals from the user terminal, and the gateway""s own received signals. The gateway commands the steps of maintaining the same information rate while increasing the coding gain andor changing at least one of the slot and the frequency of the channel in response to the threshold comparisons.